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New constraints on gamma-ray burst jet geometry and relativistic shock physics

Guidorzi, C and Mundell, CG and Harrison, R and Margutti, R and Sudilovsky, V and Zauderer, BA and Kobayashi, S and Cucchiara, A and Melandri, A and Pandey, SB and Berger, E and Bersier, D and D'Elia, V and Gomboc, A and Greiner, J and Japelj, J and Kopač, D and Kumar, B and Malesani, D and Mottram, CJ and O'Brien, PT and Rau, A and Smith, RJ and Steele, IA and Tanvir, NR and Virgili, FJ (2014) New constraints on gamma-ray burst jet geometry and relativistic shock physics. MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, 438 (1). pp. 752-767. ISSN 0035-8711

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Abstract

We use high–quality, multi-band observations of Swift GRB120404A, from γ-ray to radio frequencies, together with the new hydrodynamics code of van Eerten et al.
(2012) to test the standard synchrotron shock model. The evolution of the radio and optical afterglow, with its prominent optical rebrightening at trest � 260–2600 s, is
remarkably well modelled by a decelerating jet viewed close to the jet edge, combined with some early re–energization of the shock. We thus constrain the geometry of the
jet with half–opening and viewing angles of 23◦ and 21◦ respectively and suggest that wide jets viewed off-axis are more common in GRBs than previously thought. We also derive the fireball microphysics parameters ǫB = 2.4 × 10−4 and ǫe = 9.3 × 10−2 and a circumburst density of n = 240 cm−3. The ability to self–consistently model the microphysics parameters and jet geometry in this way offers an alternative to trying to identify elusive canonical jet breaks at late times. The mismatch between the observed and model-predicted X–ray fluxes is explained by the local rather than the global cooling approximation in the synchrotron radiation model, constraining the microphysics of particle acceleration taking place in a relativistic shock and, in turn, emphasising the need for a more realistic treatment of cooling in future developments of
theoretical models. Finally, our interpretation of the optical peak as due to the passage of the forward shock synchrotron frequency highlights the importance of high quality multi–band data to prevent some optical peaks from being erroneously attributed to the onset of fireball deceleration.

Item Type: Article
Additional Information: This is a pre-copyedited, author-produced PDF of an article accepted for publication in Monthly Notices of the Royal Astronomical Society following peer review. The version of record Guidorzi, C. et Al. (2014) New constraints on gamma-ray burst jet geometry and relativistic shock physics is available online at: http://dx.doi.org/10.1093/mnras/stt2243
Uncontrolled Keywords: 0201 Astronomical And Space Sciences
Subjects: Q Science > QB Astronomy
Divisions: Astrophysics Research Institute
Publisher: OXFORD UNIV PRESS
Related URLs:
Date Deposited: 30 Jun 2015 15:15
Last Modified: 30 Jun 2015 15:15
DOI or Identification number: /10.1093/mnras/stt2243
URI: http://researchonline.ljmu.ac.uk/id/eprint/1478

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